What are the denoising passes useful for in 2.79 experimental feature set?
There are no practical uses at the moment. It has been suggested that they will be used in compositor denoise setups, perhaps for denoising individual passes (as currently only the combined render is denoised.) However, whatever setup they are intended for has not been developed yet. It will probably happen when the denoiser is finished. So perhaps for 2.8, or perhaps not until later.

This info came from the blenderartist post. I asked about it here. I do not know if it discussed in more detail elsewhere in the thread, as the thread is huge!

https://blenderartists.org/forum/showthread.php?395313-Experimental-2-77-Cycles-Denoising-build&amp;p=3208408&amp;highlight=#post3208408